Source : Webster's Revised Unabridged Dictionary (1913)

Tankage \Tank"age\, n.
   1. The act or process of putting or storing in tanks.

   2. Fees charged for storage in tanks.

   3. The capacity or contents of a tank or tanks.

   4. (Agric.) Waste matter from tanks; esp., the dried
      nitrogenous residue from tanks in which fat has been
      rendered, used as a fertilizer.

Source : WordNet®

tankage
     n 1: the quantity contained in (or the capacity of) a tank or
          tanks
     2: the charge for storing something in tanks
     3: the act of storing in tanks